In underwater monitoring, an active sonar employing a cross fan beam method is known as a method for obtaining vertical-direction information of a target.
Such an active sonar employing the cross fan beam method which transmits transmitted fan beams, each transmitted fan beam is narrow in a vertical direction (i.e., a water depth direction) and wide in a horizontal direction. And the active sonar transmits a plurality of transmitted fan beams whose elevation angles are mutually different. And receives received fan beams, each being, reversely to the case of the transmitted fan beams, wide in the vertical direction and narrow in the horizontal direction. Such a configuration enables the active sonar to receive an echo sound having significantly high directivity from each of regions that is surrounded by vertical width of the transmitted fan beam and horizontal width of received fan beam. Further, synthesizing of such echo sound signals obtained from the respective regions enables to obtain two-dimensional information. Moreover, the visualization of this two-dimensional information enables the two-dimensional information to be utilized as a two-dimensional image, and also enables to form a three-dimensional shape of the target by using a plurality of such two-dimensional images. It is also known a method that modifies the frequency of the transmitted fan beams together with the elevation angle.
When the above-described cross fan beam method is simply used, there are some cases that accurate information cannot be obtained because of the modulations of the echo sounds or the interfusion of noises. In such background, efforts to eliminate the influence of these unfavorable factors to obtain an accurate information have been made.
For example, in patent document 1 (Japanese Unexamined Patent Application Publication No. 2015-22226), discloses a technique that corrects a frequency changes through the use of a Doppler effect. This technique enables to obtain the position and the velocity of a target by transmitting two kinds of transmitted fan beams, the one whose frequency is increasing along with the changes of its elevation angle, and the other one whose frequency is decreasing along with the changes of its elevation angle. Then the Doppler shift caused by the movement of the target is calculated, and the position and the velocity of the target can be calculated by using the Doppler shift.
Further, in patent document 2 (Japanese Unexamined Patent Application Publication No. 2012-189499) discloses a technique that can eliminate side lobes and multipath signals from a received signal through the use of the phase variance of the received signal. This technique eliminates side lobes and multipath signals by phasing a received signal, and calculating a variance of a received signal, and calculating the correlation between the phase variance and the amplitude of the received signal. Through these processes, the side lobes and the multipath echo sounds are removed. Here, the side lobes means signals that appear, as a result of beam foaming, in a direction different from the direction of a main lobe (a sound axis). And, the multipath signals means signals having arrived at a receiver after passing through a plurality of paths due to reflections or refractions.